In one way or another, most horror comes down to the internalization of an external menace or, inversely, the physical expression of internal anxieties, aversions and suspicions. Psychological fears made manifest.

Both of those notions could be true of Jennifer Kent's The Babadook, which deftly exploits those classic qualities by making its dual modes - the internal and the external, the real and the psychological - entirely codependent, and in a way that mirrors the codependency of the mother and child at the center of the story. There is no significant distinction between the main character's psyche and her physical reality, and Kent brilliantly explores the spaces within both. The film's themes, its plot details, its symbolic gestures, even the roles of its characters - Kent conflates them all, embroidering an intricate fever dream of a movie whose ideas and images all wrap around on one another.

"You'll see him if you look..."

That's just one of many unnerving lines from the strange pop-up storybook - titled Mister Babadook - from which The Babadook inherits its name, but it gets across so much about how the film operates, on both a psychological and a narrative level. Essentially, acknowledging the Babadook is what allows him (it?) to take up residence in the first place. Seeing it, feeling it, hearing it, perhaps even saying its name - that only reinforces, and strengthens, its presence. Once you let him in, as the book's text informs us, you can't get rid of the Babadook.

The book itself (which, by the way, is an extraordinary piece of design created by illustrator Alex Juhasz) is discovered atop the bookshelf one night by Samuel (Noah Wiseman) - an emotionally volatile young boy - and his mother Amelia (Essie Davis, in an outstanding portrayal of desperation transforming into lunacy), a sleep-deprived single mom trying to keep her wits about her while raising a son who's more and more frequently unhinged and prone to fits of violence. He spends much of his spare time finding and building makeshift weapons - often sneaking them into his backpack before making his way to school - that he insists he'll use to protect his mother. From what, or whom?

Kent, making one of the most impressive and assured feature debuts I've ever seen, subtly introduces certain hints and bits of backstory that blur the lines of cause and effect and make the whole film seem, especially upon reflection, like a circular portrait of madness rather than a linear storyline. There's something of an Ouroboros effect to the narrative logic. Mister Babadook - characterized by a black cloak, top hat, and long, sharp, Scissorhands-like fingers - is presented at first as an outside presence, a supernatural force trying to worm its way inside Amelia's house - or inside her mind, same difference. (The book mentions something about Mister Babadook knocking three times before being allowed in, and for the duration of the film, all of the visitors to Amelia's door knock exactly three times.) And so, the film increasingly insulates itself, holing its characters inside the house, desperate to keep this thing out. And yet within that house is the book itself - yes, she tries to destroy it, and yes, it reappears, with an even more threatening message - so Mister Babadook is ostensibly already inside. But where did the book come from? Where did Mister Babadook himself come from? Given what we know and what is implied, it's fair to ask: Is the book, in fact, the product of a poisoned mind, or is it the thing that poisoned the mind in the first place?

Particularly in the second half, we reside almost completely within that mind, within a house that's increasingly rotten and disheveled, cockroaches pouring out of the walls like the glue from Barton Fink's peeling wallpaper. The similarities to Polanski's Repulsion are easy to see.

That Kent is so economical with everything she's attempting her, and that she packs the 93-minute runtime as densely as she does, is an accomplishment in and of itself, but it's also one of the reasons why The Babadook feels so tightly wound from its opening shots. In fact, the beginning doesn't even really feel like a beginning. She thrusts us into the midst of a mother/son relationship that already feels like it's on the brink of detonation or collapse, and that's long before Mister Babadook shows up. The movie is generally linear (save for a couple of flashbacks), and has a clear beginning and end, but emotionally and atmospherically, it feels like we begin somewhere in the middle. And it all unravels from there.

So much of the best horror doesn't necessarily scare so much as it unsettles, and it unsettles because it reaches a primal place - a place where reason doesn't exist - and unleashes primal fears and anxieties. Kent already knows how to heighten that sense of anxiety, and she does it masterfully. She'll extend a moment of shrill terror or discomfort just a bit too long for us to regain our bearings. Or she'll abruptly cut off a moment at its highest emotional pitch, after holding it just long enough to keep us on edge. The extraordinary sound design is similarly distressing. Amelia and Samuel begin receiving phone calls, with the voice on the other end chanting - "Ba-ba-ba, dook! dook! dook!" - in a harsh, unnatural, cringe-inducing, back-of-the-throat screech that grows in volume until it becomes unbearable.

Kent won't let the characters off easy; they never get a respite. Earlier I mentioned Amelia's trouble getting sleep, a problem that only gets worse as the movie goes along, eventually leading to her procuring a powerful sedative for Samuel, on the hope she can get some rest without worrying about him being up in the night. Even when she does doze off, the passage of time is accelerated so that it feels like she's hardly gotten any sleep at all.

The Babadook is also a triumph of mise en scène, as the titular figure begins to materialize practically everywhere. Amelia goes to the police station and sees him in the form of a coat and hat hanging on a hook on the wall. She looks through her window to the neighbor's house and sees him, just standing there, against the darkness of a hall closet.

As strong as it is, the film begins to wobble in the third act, awkwardly rushing through one plot aspect in particular. But ultimately it lands on a marvelous epilogue, which serves as a chilling and comical punctuation mark on a movie that had already nicely straddled the line between madness and sly comedy. It also serves as a gentle reinforcement of the psychological crisis at the heart of the film. Whatever happens with the Babadook, can you ever really get rid of it?